we will use the formula: cos(A − B) = cos A cos B + sin A sin B

cos( 2π⁄3 ) cos( π⁄8 ) + sin( 2π⁄3 ) sin( π⁄8 ) = cos (2π⁄3 - π⁄8) = cos (13π/24) = - 0.1305

13π/24 rad = (13/24)*180 degrees = 97.5 degrees

cos (97.5) = - 0.1305